A time-of-flight laser rangefinder transmits a pulse of laser light and measures the time to the echo return received after reflection from the target (round trip time of flight). The most common method of determining the start, and stop or return time, is by leading edge detection of the return pulse, when the leading edge crosses a threshold value. The threshold value may be established to exclude false alarms due to noise (typically one false alarm per thousand shots).
The amplitude of the return pulse varies as a function of laser peak power, target reflectivity, atmospheric transmittance, target angle, target reflection characteristics, target size, beam divergence, and range. Some effects, such as range, may be compensated for by varying the receiver gain or threshold amplitude (such as by using time programmed gain (TPG)) as a function of time (range). Some other factors may cause two orders of magnitude variation in return signal, and resulting variations (errors) in measured range.
FIG. 1 illustrates how the amplitude of the return signal can affect range measurement. A firing pulse 102 has its leading edge threshold crossing at T0. This starts the range timing process, and the amplitude of the firing pulse 102 can be controlled. (The timing of the firing pulse can vary with different laser types, but is not dependent on external factors such as range, target. weather, etc. in T0.)
A strong return pulse 104 has its leading edge threshold crossing at T1. Range (see arrow labeled “range”) to the return pulse 104 is derived from the elapsed time between T0 and T1 (distance=velocity×time). A weaker return pulse 106 has its leading edge threshold crossing at T2. As is evident, due to the smaller amplitude of the weaker return pulse 106, T2 occurs later than T1. The later threshold crossing of the weak signal 106 will result in a longer measured range (no comparable arrow shown on drawing). This difference between apparent range (different time for same velocity implies different distance/range) of the strong and weak return pulses (signals) may be referred to as “range walk”. In other words, the amplitude of the return pulse can cause error in the range measurement.
Another source of range error is variations in the speed of light at different altitudes, which can have a direct effect on range measurement. (Again, recall that the distance measurement is based on knowing the velocity, and measuring the time.)
One technique for mitigating these errors in range measurement is to employ very short laser pulses and high bandwidth receivers. The noise of these receivers increases rapidly with bandwidth due to both the expected root bandwidth increase, and spectral noise density increase due to the detector capacitance impedance at high frequencies. The narrow laser pulse reduces the energy transmitted. As a result, it is difficult to range long distances with a small simple (such as handheld) device using this technique. The methods of economical laser construction sometimes result in longer pulses with poorer range accuracy due to range walk with the leading edge detection.
Another technique for mitigating these errors is to use a constant fraction discriminator (CFD) that measures the same part of the start (T0) and stop (return) pulse such as is disclosed in U.S. Pat. No. 6,646,479, incorporated by reference herein. This pulse discriminator technique is used in the AMI 7500 series range receiver (by Analog Technologies Inc.). In this method, the preamplifier is linear over the dynamic range of interest to preserve the pulse shape and normally results in less sensitivity due to the reduced preamplifier gain necessary to prevent saturation. The CFD solution adds complexity to a simple low cost rangefinder, and may also degrade the sensitivity due to the improved signal-to-noise ratio needed for accurate operation and detection of the laser pulse from within the background noise.
US Patent Application 20050146705, incorporated by reference herein, discloses a single-shot laser rangefinder, and uses a plurality of comparators each with its own threshold, a plurality of latches and multiple inputs to a microcontroller. The microcontroller makes range corrections as a function of signal amplitude. This approach has several disadvantages. It is complex, requiring a multi-wire interface from the comparators to the digital processor; it suffers from the sensitivity loss of the CFD solution mentioned above as the preamplifier output has to be linear in amplitude response over the dynamic range of interest; and it does not allow for the use of a variable threshold as a means of changing the sensitivity as a function of range, the simplest method of implementing TPG. As disclosed therein (Abstract):                An improvement in a single-shot laser rangefinder having a photo-detector for detecting return laser pulse signals, a signal amplifier for amplifying the return laser pulse signals, and a range processor for determining the range of a reflecting object from the round trip time of flight of the return laser pulse signals. The difference in time at which the strongest laser pulse signal crosses the threshold for detection and the weakest laser pulse signal crosses the threshold for detection causes a timing error in the measured range.        The improvement includes a plurality of comparators whose inputs are connected to the signal amplifier, each comparator outputting a digital level signal in response to an analog input signal that is more than the threshold set therein at the negative input terminal of the comparator; a plurality of latches, each latch connected to a respective comparator, the comparator outputs being fetched to the clock inputs of the latches so that when the digital level signal presents itself at the clock input, the latch then latches to the digital level signal; and a microcontroller having a plurality of inputs, each input connected to a respective latch for reading the outputs from the latches, the microcontroller having a store containing a plurality of pre-set correction factors corresponding to the range errors for various pulse amplitudes, the microcontroller having an output connected to the range processor for outputting the compensated range to the range processor upon decoding the output signals of the latches.        